If you think I tried to deny that, I'd recommend you go back and reread. What part of what I said, specifically, did you see as a denial that banks buy Treasuries for asset/reserve? The point I made is that if they're buying the Treasuries at a given rate, as partial reserve on money they'd like to lend, then the risk associated with those Treasuries goes into their calculus. They're accounting for the risk of default and devaluation. If they ascribed a high risk of either, then either they wouldn't do more loans, given the cost of buying that junk, or at least they'd lend at high rates, to offset for the risk of loss in the reserves. Right now, they're lending at some of the lowest rates in modern history.
You didn't understand the response. I'd urge you to reread. My point was that they are not required to hold dollars or treasuries to trade in oil. They could hold other currencies and other countries' debt instead, if they wanted, and occasionally that happens. Most opt for the dollar and US debt because, on balance, it offers the most advantages to do so. But, again, that suggests they're ascribing to those assets low risk, or they'd be warier about being stuck holding the bag.
Actually, if the interest rate were zero, you'd do better just to hold actual cash... there'd be no transaction cost or delay associated with liquidating it. The interest rate on Treasuries may be small, but when you're talking about billions of dollars, it's meaningful, and factors into the decision to hold Treasuries. However, it's true that the interest rate isn't very attractive at the moment. What's attractive is that, relative to nearly every other asset, it's seen as safe. It's similar to the very low interest rates on debt of several other large, wealthy nations -- even those without reserve status. For example, the UK's debt recently touched an all-time low interest rate. That's not because companies need British Pounds to buy and sell oil, and so on. It's because it's seen as a safe investment, albeit a low-earning one.